Fibrous Protein Fusions and Use Thereof in the Formation of Advanced Organic/Inorganic Composite Materials

ABSTRACT

The claimed invention provides a fusion polypeptide comprising a fibrous protein domain and a mineralization domain. The fusion is used to form an organic-inorganic composite. These organic-inorganic composites can be constructed from the nano- to the macro-scale depending on the size of the fibrous protein fusion domain used. In one embodiment, the composites can also be loaded with other compounds (e.g., dyes, drugs, enzymes) depending on the goal for the materials, to further enhance function. This can be achieved during assembly of the material or during the mineralization step in materials formation.

CROSS REFERENCED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) of U.S. provisional Patent Application No. 60/644,264, filed Jan. 14, 2005.

GOVERNMENT SUPPORT

This invention was supported by FA9550041-0363 awarded by the U.S. Air Force and EB003210-01 awarded by the National Institutes of Health. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

Many biomedical procedures require the provision of healthy tissue to counteract the disease process or trauma being treated. This work is often hampered by the tremendous shortage of tissues available for transplantation and/or grafting. Tissue engineering may ultimately provide alternatives to whole organ or tissue transplantation. In order to generate engineered tissues, various combinations of biomaterials and living cells are currently being investigated. Although attention is often focused on the cellular aspects of the engineering process, the design characteristics of the biomaterials also constitute a major challenge in this field.

In recent years, the ability to regenerate tissues and to control the properties of the regenerated tissue have been investigated by trying to specifically tune the mechanical or chemical properties of the biomaterial scaffold (Kim et al., 1997; Kohn et al. 1997). The majority of work has involved the incorporation of chemical factors into the material during processing, or the tuning of mechanical properties by altering the constituents of the material.

The foregoing methods have been used in an attempt to utilize chemical or mechanical signaling to affect changes in the proliferation and/or differentiation of cells during tissue regeneration. Despite such efforts there remains in the art a need for improved biomaterials, including composite materials, particularly those with a better capacity to support complex tissue growth in vitro (in cell culture) and in vivo (upon implantation).

SUMMARY OF THE INVENTION

The claimed invention provides a fusion polypeptide comprising a fibrous protein domain and a mineralization domain.

In one embodiment the fibrous protein domain is obtained from silk, collagens, coiled-coiled leucine zipper proteins, elastins, keratins, actins, and tubulins.

For example, in one preferred embodiment, the fibrous protein domain comprises an amino acid sequence from the silk protein Spidroin 1, such as the fibrous protein domains indicated by (SEQ ID NO: 1) or (SEQ ID NO: 3), which are derived from Spiroidin 1 of Nephila clavipes. A recombinant fibrous protein domain can be generated, which has multiple repeats of a fibrous protein domain.

In one embodiment, the fibrous domain sequence of (SEQ ID NO: 1) is repeated 15 times throughout the fibrous protein domain, which is used in the fusion proteins of the invention, i.e., the fibrous protein domain sequence indicated by (SEQ ID NO: 4), referred to herein as 15mer.

In one embodiment, a fibrous domain sequence of (SEQ ID NO: 1) is repeated 15 times throughout the fibrous protein domain that is used in the fusion proteins of the invention and has a CRGDT (SEQ ID NO: 2) linker sequence, i.e. the fibrous protein domain sequence indicated by (SEQ ID NO: 5), referred to herein as CRGD-15mer.

In one embodiment, the fusion polypeptide of the invention has a mineralizing domain that is capable of inducing the formation of hydroxyapatite, silica, cadmium sulfide or magnetite.

In one embodiment, the mineralization domain is obtained from dentin matrix protein 1 (DMP1), bone sialoprotein (BSP), or silaffin-1 (Sil1) protein.

In one embodiment, the mineralization domain is derived from dentin matrix protein 1 (DMP1) and is (SEQ ID NO: 6) or (SEQ ID NO: 7).

In one embodiment, the mineralization domain is derived from bone sialoprotein (BSP) and is (SEQ ID NO: 8).

In one embodiment, the mineralizing domain is selected from the group consisting of the 19 amino-acid R5 peptide of the Sil1 protein (SEQ ID NO: 17), the R2 peptide of the Sil1 protein (SEQ ID NO: 18), the 19 amino-acid R3 peptide of the Sil1 protein (SEQ ID NO: 19), the 19 amino-acid R6 peptide of the Sil1 protein (SEQ ID NO: 20) and the 15 amino-acid R1 peptide of the Sil1 protein (SEQ ID NO: 21).

The invention also provides for the following fusion polypeptides that comprise a fibrous protein domain and a mineralization domain: the fusion polypeptide (SEQ ID NO: 9), the fusion polypeptide (SEQ ID NO: 10), the fusion polypeptide (SEQ ID NO: 11), the fusion polypeptide (SEQ ID NO: 12), a fusion polypeptide comprising a fusion of the 15 mer silk fibrous protein domain (SEQ ID NO: 4) and the R5 peptide of the Sil1 protein (SEQ ID NO: 17) and a fusion polypeptide comprising a fusion of the CRGD-15 mer silk fibrous protein domain (SEQ ID NO: 5) and the R5 peptide of the Sil1 protein (SEQ ID NO: 17).

A method for forming a fibrous protein inorganic-composite material is also provided. The method comprises (a) contacting the fusion proteins of the invention with an inorganic material capable of mineralizing for a sufficient period of time to allow mineralization of the inorganic material.

In one preferred embodiment, the fusion proteins of the invention are formed into a silk film, foam or sponge prior to deposition of inorganic material (mineralization).

In one embodiment, the inorganic material is capable of forming hydroxyapatite or silica.

In one embodiment, the fiber, film, or sponge further comprises an agent.

In one embodiment, the inorganic coating formed on the fibrous protein comprises an agent.

In one embodiment, the agent is selected from the group consisting of a protein, peptide, nucleic acid, PNA, aptamer, antibody or a small molecule.

The claimed invention also provides for biomaterial products produced by the methods of the invention.

BRIEF DESCRIPTION OF FIGURES

FIGS. 1A and 1B show diagrammatic illustrations of fibrous protein domain-mineralization domain fusion proteins. FIG. 1A shows three claimed fusion proteins composed of a spider silk fibrous protein domain ((CRGD-15mer) (See (SEQ ID NO: 5)) and a dentin matrix protein 1 (DMP1) mineralization domain. The fusion proteins contain a mineralization domain of either 1) the full length amino acid sequence of DMP1 (See (SEQ ID NO: 7)), 2) the C-terminal end of DMP1, (See (SEQ ID NO: 6)) or a composite sequence based on DMP1, which contains the collagen binding domain 1 (CD1) of DMP1, two acidic clusters in DMP1 that are responsible for hydroxyapatite nucleation (pA and pB) and the collagen binding domain 2 (CD2) of DMP1. FIG. 1B shows a diagram of two fusion proteins, 1) fusion of a spider silk fibrous protein domain (CRGD-15mer (See (SEQ ID NO: 5)) and a bone sialoprotein mineralizing domain (BSP) (See (SEQ ID NO: 8)) and 2) fusion of a spider silk fibrous protein domain 15-mer (without CRGD linker) (See (SEQ ID NO: 4)) and a bone sialoprotein mineralizing domain (BSP) (See SEQ ID NO: 8).

FIG. 2 shows the sequence of the fusion protein CRGD-15mer-CDMP1 (SEQ ID NO: 9). CRGD-15mer-CDMP1 is a fusion protein of a spider silk fibrous protein domain ((CRGD-15mer) (See, SEQ ID NO: 5), which is indicated by a dark grey highlight, and the C-termiinal end of DMP1 mineralizing domain (See, SEQ ID NO: 6), which is indicated with light grey highlight.

FIG. 3 shows the sequence of the fusion protein CRGD-15mer-DMP1 (SEQ ID NO: 10). CRGD-15mer-DMP1 is a fusion protein of spider silk fibrous protein domain ((CRGD-15mer) (See, SEQ ID NO: 5), which is indicated by a dark grey highlight, and the full length sequence of DMP1 mineralizing domain (See, SEQ ID NO: 7), which is indicated with light grey highlight.

FIG. 4 shows the sequence of the fusion protein 15mer-BSP (SEQ ID NO: 11). 15mer-BSP is a fusion protein of spider silk fibrous protein domain ((15mer) (See, SEQ ID NO: 4), which is indicated by light grey highlight, and a mineralizing domain of bone sialoprotein (BSP) (SEQ ID NO: 8), which is indicated with dark grey highlight.

FIG. 5 shows the sequence of the fusion protein CRGD-15mer-BSP (SEQ ID NO: 12). CRGD-15mer-BSP a fusion protein of spider silk fibrous protein domain ((CRGD-15mer) (See, SEQ ID NO: 5), which is indicated by light grey highlight, and a mineralizing domain of bone sialoprotein (BSP) (SEQ ID NO: 8), which is indicated with dark grey highlight. A linker sequence between the two domains is also present.

FIGS. 6A to 6C show scanning electron microscope (SEM) images of recombinant silk film made from CRGD-15mer-CDMP1, See Example 1. FIG. 6A, image of silk film morphology prior to mineralization (HFIP+MeOH). FIG. 6B, image of silk film morphology after one round of mineralization. FIG. 6C, image of silk film morphology after three rounds of mineralization.

FIGS. 7A to 7B show the sequence of the fusion proteins CRGD15mer−R5 (SEQ ID NO: 25) and 15mer−R5 (SEQ ID NO: 25) as described in Example II. The underlined sequence represents the monomeric repeat unit selected and used in the design of the recombinant proteins based on the consensus sequence of spidroin1 (Masp1) native sequence of Nephila clavipes (Accession #P19837).

FIG. 8 shows a grid of SEM of untreated and methanol treated silk films formed from the four different genetically engineered silk proteins CRGD15mer, 15mer, CRGD15mer−R5, and 15mer−R5 as described in Example 2. Images of the control films and the films that underwent silicification reactions are shown.

FIGS. 9A to 9E show SEM images of untreated and methanol treated electropun CRGD15mer−R5 silk fibers before, during and after silicification reactions, as described in Example 2. FIG. 9A, untreated electrospun CRGD15mer−R5 silk fibers. FIG. 9B, electrospun CRGD15mer−R5 silk fibers methanol treated before silification. FIG. 9C, 9D and 9E, electrospun CRGD15mer−R5 silk fibers during silification at 20 um and 2 um scale.

FIGS. 10A to 10B show XPS analysis, as described in Example 2, of CRGD15mer−R5 and silicified CRGD15mer−R5 on Al foil and on silicon chip at the characteristic binding energies of (FIG. 10A) 153 eV and (FIG. 10B) 102 eV for electrons found in the 2s and 2p3 electron shells of the silicon atom respectively.

FIG. 11 shows Fourier transform infared spectroscopy (FTIR) analysis of the structure of CRGD-15mer-CDMP1 before and after Ca ion binding, as described in Example 1.

FIG. 12 shows FTIR analysis of structure of CRGD-15mer-CDMP1 before and after treatment with methanol, as described in Example 1.

FIGS. 13(A1) to 13(A5) and 13(B1) to 13(B5) show Scanning Electron Microscopy (SEM) surface morphologies of recombinant spider silk films after soaking in 1.5×SBF for various periods of time, as described in Example 1. FIG. 13(A1)-FIG. 13(A5), SS15m-CDMP1 soaked in 1.5×SBF for 0, 3, 7, 14 and 21 days: FIG. 13(A1)-FIG. 13(A5) respectively. FIG. 13(B1)-FIG. 13(B5), SS15 m soaked in 1.5×SBF for 0, 3,7,14 and 21 days: FIG. 13(B1)-FIG. 13(B5) respectively. Scale bars are 2 μm.

FIGS. 14A to 14C show Transmission Electron Microscopy (TEM) images of crystals grown on CRGD-15mer-CDMP1 after soaking in 1.5×SBF for 21 days, as described in Example 1. (FIG. 14A) image of flake-like apatite; (FIG. 14B) electron diffraction pattern; (FIG. 14C) high-resolution image of apatite crystal.

DESCRIPTION OF THE INVENTION

Fibrous proteins represent an important category of proteins in biology, forming native structural entities both internally in organisms (e.g., collagens) and externally (such as spun silk fibers for webs and cocoons). As an example, silk proteins from spiders and insects provide a number of valuable materials features. When combined with functional domains, then the materials properties available from silks become significantly expanded. In accordance with the present invention, various domains from biology known to promote the formation of inorganic structures, e.g., promote formation of minerals, have been combined with a fibrous protein domain. Thus, these new fusion (composite) materials offer the benefits of the fibrous protein material features (e.g, silks) plus the benefit of the inorganic mineralization domains (e.g., hardness).

In one embodiment, the present invention provides a fusion polypeptide comprising a fibrous protein domain and a mineralization domain. The fusion is used to form an organic-inorganic composite. These organic-inorganic composites can be constructed from the nano- to the macro-scale depending on the size of the fibrous protein fusion domain used. In one embodiment, the composites can also be loaded with other compounds (e.g., dyes, drugs, enzymes) depending on the goal for the materials, to further enhance function. This can be achieved during assembly of the material or during the mineralization step in materials formation.

The fusion polypeptides of the present invention can be used for production of silk biomaterials, e.g., fibers, films, foams and mats. See, WO 03/022909. An all-aqueous process may be used. See, WO 03/022909.

In another embodiment, the invention provides a method for forming a composite material. The method comprises contacting a template formed from a fusion polypeptide of the invention with a suitable inorganic material for a sufficient period of time to allow mineralization of the inorganic material thus forming an inorganic coating on the template.

In one embodiment the template is in the form of a fiber, film, or sponge.

In an additional embodiment, growth factors, biological components or others agents, including therapeutic agents, are incorporated into the inorganic coating. The growth factors, biological components or other agents, can be incorporated during the formation of the template or during the crystallization process. Such composites can be used to deliver such components to cells and tissues. The ability to incorporate growth factors, biological regulators, enzymes, therapeutics, or cells in the construct of the present invention provides for stabilization of these components for long term release and stability, as well as better control of activity and release.

The products produced by these methods offer new options in the formation of scaffolds for biomaterials, tissue engineering applications and drug delivery. While the templates are useful in and of themselves, the ability to form inorganic coatings with controlled thickness leads to control of mechanical properties (e.g., stiffness) and biological interactions, such as for bone formation. Furthermore, the ability to control these processes allows one to match structural and functional performance of scaffolds for specific tissue targets and needs.

In another embodiment, the underlying template (e.g., fusion polypeptide formed into a film etc.) can be removed or etched away to generate porous networks, tubes, or lamellar sheets of inorganic material. These materials are useful directly as biomaterial scaffolds, for control of cell and tissue growth (e.g., as nerve conduits, bone conduits) and for nonbiological applications (e.g., filtration and separation media, catalysis, decontamination (directly or if filled with appropriate chemical or enzymes), radar chaff, coatings in general, and many related needs, for example, inorganic fillers to toughen materials that can also be filled with a second component.

The fusion polypeptides may be created, for example, by chemical synthesis, or by creating and translation a polynucleotide in which the peptide regions are encoded in the desired relationship.

It is to be understood that while the invention has been described in conjunction with the preferred specific embodiments thereof that the foregoing description as well as the examples that follow are intended to illustrate and not limit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains.

Fibrous Protein Domains

Silks are unique in their self-assembly, formation of robust materials in the form of fibers, films and 3D matrices, and are polymorphic (the structure can be controlled among many assembly states). For example, silk fibers are the strongest known fibers and rival even high performance fibers. They are also effective in resisting compression. In 3D porous matrices the compressive resistance exceeds other commonly used organic polymers as biomaterial matrices. Inorganic domains, including silica and hydroxyapatite are prominent in biological systems as key inorganic components found associated with proteins. These mineral phases dominate skeletal components of most systems.

The fibrous protein domain can include short to long versions, with the size in part influencing the scale of the composite from the nano to the macro level. For example, the size can range from a single sized hydrophobic block of 50 amino acids, up to multiple blocks as large as desired including up to proteins of sizes in the 100,000s of Daltons. In addition, the fibrous protein fusion domain could include other proteins such as collagens, coiled-coiled leucine zipper proteins, elastins, keratins, actins, and tubulins.

The silk protein suitable for use in the present invention is preferably fibroin or related proteins (i.e., silks from spiders). The silkworm silk is obtained, for example, from Bombyx mori. Spider silk may be obtained from Nephila clavipes See, for example, WO 97/08315 and U.S. Pat. No. 5,245,012.

Inorganic Domains/Mineralizing Domains

Mineralizing domains include those that can induce the formation of hydroxyapatite (Example 1), silica (Example 2), cadmium sulfide, and magnetite. In one embodiment, a mineralizing domain that can induce hydroxyapatite nucleation is obtained from dentin matrix protein 1 or bone sialoprotein (G. He, T. Dahl, A. Veis and A. George. Connective Tissue Res. 2003, 44 (Suppl. 1): 240-245; and G. He, T. Dahl, A. Veis and A. George. Nature materials. 2003, 2(8): 552-558; Stubbs et al. J Bone Miner Res. 1997 Aug;12(8):1 210-22). The full molecule or a minimum portion that can induce mineralization can be used, for example, a 37 amino acid segment from DMP (C-terminus) will induce controlled mineralization of hydroxyapatite. In one example, the mineralization domains were fused to a sequence derived from spider silk having the following silk fibrous domain repeated 15 times (SGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGT) (SEQ ID NO: 1) in order to generate functional recombinant silks that possess both robust mechanical properties of the spider silk and the ability to promote mineralization. The recombinant proteins were cloned in pET21a(+) vector and expressed in E. coli. As demonstrated below, the identities of these recombinant silks were confirmed by amino acid composition analysis and the mineralization study was carried out on the surfaces of the cast functional recombinant silk films using recombinant protein containing only spider silk sequence as control. It was demonstrated that the functional recombinant silks exhibit an ability to promote the nucleation of hydroxyapatite. The functional recombinant silks (silk fusion proteins) of the present invention have potential application in biomaterials, tissue engineering, advanced material composites and biosensors.

Preferably, the mineralizing domain is capable of inducing the formation of hydroxyapatite, silica, cadmium sulfide, magnetite and other metal salts pending choice of peptide domain utilized.

Preferred mineralizing domains include, for example, dentin matrix protein 1 (DMP1), bone sialoprotein, and fragments of these proteins, and the 19 amino-acid R5 peptide of the Sil1 protein.

Alternative fibrous proteins and mineralizing domains can also be included in this system using the template provided. For example, peptides identified from other native proteins or identified by combinatorial screening methods can be used. Screening methods can involve any mineralization assay known in the art, for example the hydroxyapatite and silicification assays described herein.

Preparation of Fusion Proteins; Vectors, Host Cells and Expression

The fusion proteins containing a mineralization domain and a fibrous protein domain can be made using standard molecular biology methods well known to those skilled in the art (See for example, Sambrook et al., Molecular Biology: A laboratory Approach, Cold Spring Harbor, N.Y. 1989; Ausubel, et al., Current protocols in Molecular Biology, Greene Publishing, Y, 1995).

In some embodiments, the fusion proteins of the invention are made using linker sequences. As used herein, the term “linker sequence” refers to a short (e.g., about 1-20 amino acids) sequence of amino acids that is not part of the sequence of either of two polypeptides being joined. For example, a linker sequence is attached on its amino-terminal end to one polypeptide or polypeptide domain and on its carboxyl-terminal end to another polypeptide or polypeptide domain.

The manipulation of nucleic acids that encode the protein domains in the present invention is typically carried out in recombinant vectors. Herein, both phagemid and non-phagemid vectors can be used. As used herein, vector refers to a discrete element that is used to introduce heterologous DNA into cells for the expression and/or replication thereof. Methods by which to select or construct and, subsequently, use such vectors are well known to one of skill in the art. Numerous vectors are publicly available, including bacterial plasmids, bacteriophage, artificial chromosomes, episomal vectors and gene expression vectors. A vector of use according to the invention may be selected to accommodate a polypeptide coding sequence of a desired size. A suitable host cell is transformed with the vector after in vitro cloning manipulations. Host cells may be prokaryotic, such as any of a number of bacterial strains, or may be eukaryotic, such as yeast or other fungal cells, insect or amphibian cells, or mammalian cells including, for example, rodent, simian or human cells. Each vector contains various functional components, which generally include a cloning (or “polylinker”) site, an origin of replication and at least one selectable marker gene. If given vector is an expression vector, it additionally possesses one or more of the following: enhancer element, promoter, transcription termination and signal sequences, each positioned in the vicinity of the cloning site, such that they are operatively linked to the gene encoding a polypeptide repertoire member according to the invention.

Both cloning and expression vectors generally contain nucleic acid sequences that enable the vector to replicate in one or more selected host cells. Typically in cloning vectors, this sequence is one that enables the vector to replicate independently of the host chromosomal DNA and includes origins of replication or autonomously replicating sequences. Such sequences are well known for a variety of bacteria, yeast and viruses. For example, the origin of replication from the plasmid pBR322 is suitable for most Gram-negative bacteria, the 2 micron plasmid origin is suitable for yeast, and various viral origins (e.g. SV 40, adenovirus) are useful for cloning vectors in mammalian cells. Generally, the origin of replication is not needed for mammalian expression vectors unless these are used in maminalian cells able to replicate high levels of DNA, such as COS cells.

Advantageously, a cloning or expression vector may contain a selection gene also referred to as a selectable marker. This gene encodes a protein necessary for the survival or growth of transformed host cells grown in a selective culture medium. Host cells not transformed with the vector containing the selection gene will therefore not survive in the culture medium. Typical selection genes encode proteins that confer resistance to antibiotics and other toxins, e.g. ampicillin, neomycin, methotrexate or tetracycline, complement auxotrophic deficiencies, or supply critical nutrients not available in the growth media.

Since the replication of vectors according to the present invention is most conveniently performed in E. coli (e.g., strain TB1 or TG1), an E. coli-selectable marker, for example, the β-lactamase gene that confers resistance to the antibiotic ampicillin, is of use. These can be obtained from E. coli plasmids, such as pBR322 or a pUC plasmid such as pUC18 or pUC19, or pUC119.

Expression vectors usually contain a promoter that is recognized by the host organism and is operably linked to the coding sequence of interest. Such a promoter may be inducible or constitutive. The term “operably linked” refers to a juxtaposition wherein the components described are in a relationship permitting them to function in their intended manner. A control sequence “operably linked” to a coding sequence is ligated in such a way that expression of the coding sequence is achieved under conditions compatible with the control sequences.

Promoters suitable for use with prokaryotic hosts include, for example, the β-lactamase and lactose promoter systems, alkaline phosphatase, the tryptophan (trp) promoter system and hybrid promoters such as the tac promoter. Promoters for use in bacterial systems will also generally contain a Shine-Delgarno sequence operably linked to the coding sequence. Preferred promoters for use in the present invention are the isopropylthiogalactoside (IPTG)-regulatable promoters.

In one preferred embodiment of the invention, the fusion protein of the invention further comprises a tag, e.g. Flag, His, Myc, HA, VSV, or V5, to aid in purification of the protein by standard means. After purification, the fusion protein can be lyophilized or suspended in aqueous solution for preparation of silk materials, such as films, foams or fibers.

Formation of Silk Fibers, Films, Foams and Gels using the Silk Fusion Protein of the Invention

The silk fusion proteins of the invention can be processed into films, foams or fibers. As used herein, “silk fibroin” or “silk protein” refers to a silk fusion protein of the invention.

Fibers can be formed by electrospinning. Electrospinning can be performed by any means known in the art (see, for example, U.S. Pat. No. 6,110,590). Preferably, a steel capillary tube with a 1.0 mm internal diameter tip is mounted on an adjustable, electrically insulated stand. Preferably, the capillary tube is maintained at a high electric potential and mounted in the parallel plate geometry. The capillary tube is preferably connected to a syringe filled with silk/biocompatible polymer solution. Preferably, a constant volume flow rate is maintained using a syringe pump, set to keep the solution at the tip of the tube without dripping. The electric potential, solution flow rate, and the distance between the capillary tip and the collection screen are adjusted so that a stable jet is obtained. Dry or wet fibers are collected by varying the distance between the capillary tip and the collection screen.

A collection screen suitable for collecting silk fibers can be a wire mesh, a polymeric mesh, or a water bath. Alternatively and preferably, the collection screen is an aluminum foil. The aluminum foil can be coated with Teflon fluid to make peeling off the silk fibers easier. One skilled in the art will be able to readily select other means of collecting the fiber solution as it travels through the electric field. As is described in more detail in the Examples section below, the electric potential difference between the capillary tip and the aluminum foil counter electrode is, preferably, gradually increased to about 12 kV, however, one skilled in the art should be able to adjust the electric potential to achieve suitable jet stream.

The process of the present invention may further comprise steps of immersing the spun fiber into an alcohol/water solution to induce crystallization of silk. The composition of alcohol/water solution is preferably 90/10 (v/v). The alcohol is preferably methanol, ethanol, isopropyl alcohol (2-propanol) or n-butanol. Methanol is most preferred. Additionally, the process may further comprise the step of washing the fibroin fiber in water.

In another embodiment, the biomaterial is a film. The process for forming the film comprises, for example, the steps of (a) preparing an aqueous silk fibroin solution comprising silk protein; (b) adding a biocompatible polymer to the aqueous solution; (c) drying the mixture; and (d) contacting the dried mixture with an alcohol (preferred alcohols are listed above) and water solution to crystallize a silk blend film. Preferably, the biocompatible polymer is poly(ethylene oxide) (PEO). The process for producing the film may further include step (e) of drawing or mono-axially stretching the resulting silk blend film to alter or enhance its mechanical properties. The stretching of a silk blend film induces molecular alignment in the fiber structure of the film and thereby improves the mechanical properties of the film.

In a further embodiment, the biomaterial is a foam. Foams may be made from methods known in the art, including, for example, freeze-drying and gas foaming in which water is the solvent or nitrogen or other gas is the blowing agent, respectively.

In one embodiment, the foam is a micropatterned foam. Micropatterned foams can be prepared using, for example, the method set forth in U.S. Pat. No. 6,423,252, the disclosure of which is incorporated herein by reference.

Formation of Organic-Inorganic Composites

The claimed invention provides a method for forming an organic-inorganic composite material. The method comprises contacting a template (such as recombinant silk films, sponges or fibers) formed from a fusion polypeptide of the invention with a suitable inorganic material for a sufficient period of time to allow mineralization of the inorganic material. Mineralized material is deposited onto the silk forming an inorganic coating on the template.

The procedure for mineralization of the coating is dependent upon the mineralization domain that is part of the fusion polypeptide of the invention. For example, the mineralizing domain of dentin matrix protein 1 (DMP1), an acidic phosphoprotein secreted into the extracellular matrix during the formation and mineralization of bone and dentin, is involved in the precipitation of hydroxyapatite, the main inorganic component in calcified hard tissue such as bone and teeth of vertebrates (Koutsopoulos, S. J Biomed Mater Res. 2002, 62: 600-612). Further, the mineralizing domain of Silaffin protein is involved in the precipitation of silica.

Those in the art are skilled to select the appropriate buffer for precipitation of inorganic material. The formation of inorganic coatings is further described in Examples 1 and 2.

The biomaterial products produced by the processes of the present invention may be used in a variety of medical applications such as wound closure systems, including vascular wound repair devices, hemostatic dressings, patches and glues, sutures, drug delivery and in tissue engineering applications, such as, for example, scaffolding, ligament prosthetic devices and in products for long-term or bio-degradable implantation into the human body. A preferred tissue engineered scaffold is a non-woven network of electrospun fibers.

Additionally, these biomaterials can be used for organ repair replacement or regeneration strategies that may benefit from these unique scaffolds, including but are not limited to, spine disc, cranial tissue, dura, nerve tissue, liver, pancreas, kidney, bladder, spleen, cardiac muscle, skeletal muscle, tendons, ligaments and breast tissues.

In another embodiment of the present invention, silk biomaterials can contain therapeutic agents. To form these materials, the polymer would be mixed with a therapeutic agent prior to forming the material or loaded into the material after it is formed. In addition, the agent can be incorporated into the inorganic coating formed by the silk fusion protein mineralization domain by mixing it with the mineralization solution. The variety of different therapeutic agents that can be used in conjunction with the biomaterials of the present invention is vast. In general, therapeutic agents which may be administered via the pharmaceutical compositions of the invention include, without limitation: antiinfectives such as antibiotics and antiviral agents; chemotherapeutic agents (i.e. anticancer agents); anti-rejection agents; analgesics and analgesic combinations; anti-inflammatory agents; hormones such as steroids; growth factors (bone morphogenic proteins (i.e. BMP's 1-7), bone morphogenaic-like proteins (i.e. GFD-5, GFD-7 and GFD-8), epidermal growth factor (EGF), fibroblast growth factor (i.e. FGF 1-9), platelet derived growth factor (PDGF), insulin like growth factor (IGF-I and IGF-II), transforming growth factors (i.e. TGF-.beta.I-III), vascular endothelial growth factor (VEGF)); and other naturally derived or genetically engineered proteins, polysaccharides, glycoproteins, or lipoproteins. These growth factors are described in The Cellular and Molecular Basis of Bone Formation and Repair by Vicki Rosen and R. Scott Thies, published by R. G. Landes Company hereby incorporated herein by reference.

Silk biomaterials containing bioactive materials may be formulated by mixing one or more therapeutic agents with the polymer used to make the material. Alternatively, a therapeutic agent could be coated on to the material preferably with a pharmaceutically acceptable carrier. Any pharmaceutical carrier can be used that does not dissolve the foam. The therapeutic agents, may be present as a liquid, a finely divided solid, or any other appropriate physical form. Typically, but optionally, the matrix will include one or more additives, such as diluents, carriers, excipients, stabilizers or the like.

The amount of therapeutic agent will depend on the particular drug being employed and medical condition being treated. Typically, the amount of drug represents about 0.001 percent to about 70 percent, more typically about 0.001 percent to about 50 percent, most typically about 0.001 percent to about 20 percent by weight of the material. Upon contact with body fluids the drug will be released.

The biocompatible polymer may be extracted from the biomaterial prior to use. This is particularly desirable for tissue engineering applications. Extraction of the biocompatible polymer may be accomplished, for example, by soaking the biomaterial in water prior to use.

The tissue engineering scaffolds biomaterials can be further modified after fabrication. For example, the scaffolds can be coated with bioactive substances that function as receptors or chemoattractors for a desired population of cells. The coating can be applied through absorption or chemical bonding.

Additives suitable for use with the present invention include biologically or pharmaceutically active compounds. Examples of biologically active compounds include cell attachment mediators, such as the peptide containing variations of the “RGD” integrin binding sequence known to affect cellular attachment, biologically active ligands, and substances that enhance or exclude particular varieties of cellular or tissue ingrowth. Such substances include, for example, osteoinductive substances, such as bone morphogenic proteins (BMP), epidermal growth factor (EGF), fibroblast growth factor (FGF), platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), insulin-like growth factor (IGF-I and II), TGF- and the like.

The scaffolds are shaped into articles for tissue engineering and tissue guided regeneration applications, including reconstructive surgery. The structure of the scaffold allows generous cellular ingrowth, eliminating the need for cellular preseeding. The scaffolds may also be molded to form external scaffolding for the support of in vitro culturing of cells for the creation of external support organs.

The scaffold functions to mimic the extracellular matrices (ECM) of the body. The scaffold serves as both a physical support and an adhesive substrate for isolated cells during in vitro culture and subsequent implantation. As the transplanted cell populations grow and the cells function normally, they begin to secrete their own ECM support.

In the reconstruction of structural tissues like cartilage and bone, tissue shape is integral to function, requiring the molding of the scaffold into articles of varying thickness and shape. Any crevices, apertures or refinements desired in the three-dimensional structure can be created by removing portions of the matrix with scissors, a scalpel, a laser beam or any other cutting instrument. Scaffold applications include the regeneration of tissues such as nervous, musculoskeletal, cartilaginous, tendenous, hepatic, pancreatic, ocular, integumenary, arteriovenous, urinary or any other tissue forming solid or hollow organs.

The scaffold may also be used in transplantation as a matrix for dissociated cells, e.g., chondrocytes or hepatocytes, to create a three-dimensional tissue or organ. Any type of cell can be added to the scaffold for culturing and possible implantation, including cells of the muscular and skeletal systems, such as chondrocytes, fibroblasts, muscle cells and osteocytes, parenchymal cells such as hepatocytes, pancreatic cells (including Islet cells), cells of intestinal origin, and other cells such as nerve cells, bone marrow cells, skin cells and stem cells, and combination thereof, either as obtained from donors, from established cell culture lines, or even before or after genetic engineering. Pieces of tissue can also be used, which may provide a number of different cell types in the same structure.

The cells are obtained from a suitable donor, or the patient into which they are to be implanted, dissociated using standard techniques and seeded onto and into the scaffold. In vitro culturing optionally may be performed prior to implantation. Alternatively, the scaffold is implanted, allowed to vascularize, then cells are injected into the scaffold. Methods and reagents for culturing cells in vitro and implantation of a tissue scaffold are known to those skilled in the art.

The biomaterials of the claimed invention may be sterilized using conventional sterilization process such as radiation based sterilization (i.e. gamma-ray), chemical based sterilization (ethylene oxide) or other appropriate procedures. Preferably the sterilization process will be with ethylene oxide at a temperature between 52-55° C. for a time of 8 hours or less. After sterilization the biomaterials may be packaged in an appropriate sterilize moisture resistant package for shipment and use in hospitals and other health care facilities.

The invention will be further characterized by the following examples which are intended to be exemplary of the invention.

EXAMPLES Example 1 Formation of Hydroxyapatite Silk Fusions

Hydroxyapatite (HAP, Ca₅(PO₄)₃(OH)) is the most stable calcium phosphate salt at normal temperature and pH between 4 and 12 (Aaron, S. Posner Physiological review, 1969, 49(4): 760-792). Hydroxyapatite is the main inorganic component in calcified hard tissue such as bone and teeth of vertebrates (Koutsopoulos, S. J Biomed Mater Res. 2002, 62: 600-612). In addition, hydroxyapatite also has many applications in protein chromatography, water treatment processes, fertilizer and pharmaceutical products (Bailliez, S.; Nzihou, A.; Beche, E.; Flamant, G. Process Safety and Environmental Protection, 2004, 82(B2): 175-180 ). For calcified hard tissue, hydroxyapatite contributes to its stiffness, while organic matrix contributes to its plasticity (Landis, W. J. Bone, 16(5): 533-544). The inherent strength and other mechanical properties of the skeletal system are thought to depend on an interaction between its organic and inorganic matrix strength, here hydroxyapatite. One example is the composite formed between the normal mineral salt, hydroxyapatite, and collagen, the principal organic component of the vertebrate skeleton, able to resist a wide range of compressive or tensile forces whereas either material alone can not (Chen, Q. Z.; Wong, C. T.; Lu, W. W.; Cheung, K. M. C.; Leong, J. C. Y. and Luk, K. D. K. Biomaterials, 2004, 25: 4243-4254). In the present disclosure, we demonstrate a proof of concept with dental matrix protein (DMP).

Dentin matrix protein 1 (DMP1) is an acidic phosphoprotein secreted into the extracellular matrix during the formation and mineralization of bone and dentin, therefore, it is believed that DMP1 plays an important role in the initiation of mineralization (J. Q. Feng, H. Huang, Y. Lu, L. Ye, Y. Xie, T. w. Tsutsui, T. Kunieda, T. Castranio, G. Scott, L. B. Bonewald, and Y. Mishina. J Dent Res. 2003, 82(10): 776-780; W. T. Butler, H. Ritchie. Int J Dev Biol, 1995, 39: 169-179; George, B. Sabsay, P.A.L. Simonian, and A. Veis. J Biol Chem, 1993, 268(17):12624-12630). It has been reported that DMP1 nucleates the formation of hydroxyapatite in vitro (G. He, T. Dahl, A. Veis and A. George. Connective Tissue Res. 2003, 44 (Suppl. 1): 240-245; and G. He, T. Dahl, A. Veis and A. George. Nature materials. 2003, 2(8): 552-558) by binding calcium ions and initiating mineral deposition. It has also been reported that DMP1 binds to the N-telopeptide region of type I collagen and the collagen-binding domains involved in this interaction have been identified (G. He and A. George. J Biol Chem. 2004, 279(12): 11649-11656).

We generated various fibrous protein domain-mineralizing domain fusion proteins. See for example FIGS. 1-5. FIG. 2 shows the sequence of CRGD-15mer-CDMP1, a fusion protein of a spider silk fibrous protein domain ((CRGD-15mer) (See, SEQ ID NO: 5), which is indicated by a dark grey highlight, and the C-terminal end of DMP1 mineralizing domain (See, SEQ ID NO: 6), which is indicated with light grey highlight.

FIG. 3 shows the sequence of the fusion protein CRGD-15mer-DMP1 (SEQ ID NO: 10). CRGD-15mer-DMP 1 is a fusion protein of spider silk fibrous protein domain ((CRGD-15mer) (See, SEQ ID NO: 5), which is indicated by a dark grey highlight, and the full length sequence of DMP1 mineralizing domain (See, SEQ ID NO: 7), which is indicated with light grey highlight.

FIG. 4 shows the sequence of the fusion protein 15mer-BSP (SEQ ID NO: 11). 15mer-BSP is a fusion protein of spider silk fibrous protein domain ((15mer) (See, SEQ ID NO: 4), which is indicated by light grey highlight, and a mineralizing domain of bone sialoprotein (BSP) (SEQ ID NO: 8), which is indicated with dark grey highlight.

FIG. 5 shows the sequence of the fusion protein CRGD-15mer-BSP (SEQ ID NO: 12). CRGD-15mer-BSP a fusion protein of spider silk fibrous protein domain ((CRGD-15mer) (See, SEQ ID NO: 5), which is indicated by light grey highlight, and a mineralizing domain of bone sialoprotein (BSP) (SEQ ID NO: 8), which is indicated with dark grey highlight.

Construction of Expression Vector for Recombinant Spider Silks: The consensus repeat unit of spider silk (-SGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGT-) (SEQ ID NO: 1) was derived from the native sequence of the spidroin 1 of N. clavipes (accession P19837). The construction of the expression vector pET30a (+) containing 15 repeats of the silk was previously described (Bini et al., 2005) and was termed pET30a (+)-15mer. Plasmid containing the rat dentin matrix protein 1 cDNA (pGEX-DMP1) was previously described (Feng et al., J. Dent. Res. 2003, 82(10):776-780). Primers with BamHI and XhoI restriction enzyme sites were designed in order to copy the C-terminal portion of rat DMP1 from pGEX-DMP1 by PCR: BamHI site, C-DMP1f: 5′-CAGGATCCAGGGGTGACAACCCAGAT-3′ (SEQ ID NO: 26) and Xho I site, C-DMP1r: 5′-GCCTCGAGGTAGCCATCTTGGCAATC-3′ (SEQ ID NO: 27).

PCR products were double digested with BamHI and XhoI before running on a 1% agarose gel. The band of C-terminal DMP1 DNA were cut from gel and purified using a MinElute gel extraction kit. Expression vector pET21a (+) was also double digested with BamHI and XhoI before running on a 0.8% agarose gel. The band of linearized pET21a (+) was purified using a QIAquick gel extraction kit. After determining the DNA concentration of the vector pET21a (+) and insert of C-terminal DMP1 based on intensities of 1 μl of each in a 1% gel against 1 μl of high mass DNA ladder, the ligation reactions were performed using a Quick ligation kit™ based on an insert to vector molar ratio of 5:1. Fifty μl of one shot® Mach1™ T1 phage-resistant cells were transformed with 1 μl of ligation reaction. The correct clone of vector pET21a (+) containing C-terminal DMP1, named as pET21a (+)-CDMP1 was determined by DNA sequencing with forward and reverse T7 primers. pET21a (+)-CDMP1 was digested with BamHI and dephosphorylated with CIP before running on 0.8% agarose gel. The linearized vector was purified using a QIAquick gel extraction kit. pET30a (+)-15mer was digested with BamHI before running on 1% agarose gel and the DNA encoding the 15 repeat unit of spider silk was purified using a MinElute gel extraction kit. The DNA concentrations of the vector pET21a (+)-CDMP1 and the insert: spider silk 15 mer were determined using the same method as described above. The spider silk 15 mer was then inserted into pET21a (+)-CDMP1 at the BamHI site by ligation reaction and the resulting vector was named as pET21a (+)-SS15m-CDMP1. DNA sequencing with forward and reverse T7 primers was performed to check the sequences of the vector.

Protein Expression and Purification: Expression plasmid pET21a (+)-SS15m-CDMP1 was transformed into Escherichia coli RY-3041 strain, kindly provided by Professor Ry Young (Texas A&M University, College Station, Tex). High cell density cultivation using a 1.25 liter jar fermentor (Bioflo 3000; New Brunswick Scientific Co., Edison, N.J.) was performed as described previously (Butler et al. Int. J. Dev. Biol. 1995, 39: 169-179). Expression was induced by adding isopropyl-β-D-thiogalactoside to a final concentration of 2 mM at the early log phase when A₆₀₀ was about 30. Cells were harvested after 3 h of induction by centrifuging the culture at 4° C., 6000×g, for 10 min. The cells were then sonicated on ice using sonicator equipped with a microtip. Ten second burst at 100 W with a 10 second cooling period between each burst was applied. The lysate was centrifuged at 10,000×g for 30 min at 4° C. to pellet the cellular debris. The recombinant silk protein was purified using Ni-NTA agarose resin from supernatant at 4° C. The purification fractions that contain the target protein were dialyzed against distilled water for 2 days and dry proteins were prepared by lyophilizing the aqueous solution. Dialyzed purification fractions were also concentrated and desalted by using Centricon plus 70 (NMW=10 kDa).

Protein Characterization: SDS-PAGE was performed to analyze purification fractions and Western blot was performed using His-tag monoclonal antibody to further confirm the expression of the target proteins. Western blot analysis of CRGD-15mer-CDMP1 showed expression of the protein (data not shown). The target protein bands on 4-12% Bis-Tris PAGE gel were analyzed at the Yale University W. M. Keck Biotechnology Resource Laboratory to determine amino acid compositions. A Bruker Proflex™ mass spectrometer (Bruker, Billerica, Mass.) was used for molecular weight determination.

Change of Protein Secondary Structure by Calcium Ion Binding: 0.2 mg/mL protein aqueous solution was prepared by the method described in protein expression and purification. Calcium chloride solution (1M) was added such that a molar ratio of protein to Ca ion was maintained at 1:1000. Protein secondary structures before and after adding Ca ion were studied by Fourier Transform Infrared Spectroscopy (FTIR). FTIR studies were performed using a Bruker Tensor 27 FTIR spectrometer with a BioATR accessory.

Film Formation: Recombinant proteins were dissolved in hexafluoroisopropanol (HFIP) to a final concentration of 2% w/v. One-hundred μl of silk HFIP solution was loaded onto a silica wafer and dried in hood. Dried silk film was treated with 90% methanol to induce the transition of secondary structure from random coil to beta sheet. Concentrated silk aqueous solution was then loaded onto silk film and incubated at 37° C. overnight to dry.

In virto Mineralization: The silk films formed on silica wafer were then incubated in 1.5× simulated body fluid (SBF). 1.5×SBF was prepared as described previously (3) by dissolving reagent grade CaCl₂, KH₂PO₄, NaCl, KCl, MgCl₂.6H₂O, NaHCO₃, Na₂SO₄ in distilled water and buffering at pH 7.3 with tris-hydroxymethyl aminomethane and hydrochloric acid (HCl). A 1.5×SBF solution contains 1.5 times higher ion concentration than the SBF solution with ion concentrations close to human blood plasma. Fresh 1.5×SBF was prepared daily to replace the 1.5×SBF. After the silk coated silica wafers were soaked in 1.5×SBF for 3, 7, 14 and 21 days, they were removed, gently rinsed with distilled water and dried at room temperature.

Scanning Electron Microscopy: Morphological investigation of the dried films before and after incubating in 1.5×SBF was performed using LEO 982 Field Emission Scanning Electron Microscope (SEM) (LEO Electron Microscopy, Inc., Thornwood, N.Y.) at 3.0 kV. The sample was sputter coated with gold prior to examination.

Transmission Electron Microscopy: Samples for transmission electron microscopy (TEM) analysis were prepared by ultrasonicating of the silk films after incubation in 1.5×SBF in 200 μL of absolute ethanol for 10 min and then depositing a few drops of the suspension onto a standard TEM copper grid with a holey carbon support film. TEM images were obtained with a JOEL 2100 TEM operating at 200 kV with a LaB₆ filament and recorded with a slow scan CCD camera. The diffraction patterns were obtained at calibrated camera lengths using a NiO_(x) test specimen as a reference.

Results

Protein Expression and Purification: CRGD-15mer, molecular weight of which is 48.56 kDa, migrated to the right position on SDS-PAGE gel, while the apparent molecular weight of CRGD-15mer-CDMP1 indicated by SDS-PAGE electrophoresis was higher than the calculated molecular weight, which is 58.89 kDa. This is due to high content of acidic amino acids such as aspartic acid and glutamic acid in CRGD-15mer-CDMP1. Western blot analysis of CRGD-15mer-CDMP1 and CRGD-15mer-CDMP1 was detected based on the His tag (data not shown).

Amino Acid Composition Analysis: Amino acid composition analysis confirmed the correct composition for the purified CRGD-15mer-CDMP1.

Functional Change of Protein Secondary Structure by Calcium Ion Binding: CRGD-15mer-CDMP1 was unordered random coil in water before adding CaCl₂, indicated by amide 1 peak at 1649 cm⁻¹ and amide II peak at 1548 cm⁻¹. After Ca ion binding, α helix and β sheet structures showed up, demonstrated by amide I peak at 1659 cm⁻¹ (α helix) and 1632 cm⁻¹ (β sheet) (FIG. 11).

Film Formation: Silk secondary structure changed from random coil to β sheet after treated with 90% methanol (FIG. 12) so that the film was more robust in solution.

Functional Assessments of the Fusion Proteins

The apatite nucleation ability of the functional recombinant silks was studied using the method of alternate soaking process. Silk films of the recombinant fusion proteins were cast using standard methods. Lyophilized fusion proteins were dissolved in HFIP solvent at a concentration of 2.5% w/v overnight at 4° C. The silk-HFIP solutions were then pipetted into 24-well culture plates and left in the hood for about 3 hours for the silk films to form and dry out. The silk films were then treated with 90% v/v methanol to induce β-sheet formation and prevent resolubilization of the films in aqueous solutions.

Scanning Electron Microscopy: To asses the ability for mineralization, the cast recombinant silk film was first soaked in 200 mM aqueous calcium chloride solution buffered with tris(hydroxymethyl) aminomethane and HCl (pH 7.4) (Ca solution) for 1 hr at 37° C., then Ca solution was aspirated and the film was washed with abundant distilled water to wash way any unbound Ca²⁺, then 120 mM aqueous disodium hydrogenphosphate (P solution) was added to immerse the silk film for 1 hr at 37° C. After soaking in P solution for 1 hr, the film was washed again by distilled water. This is one round of mineralization. Three rounds of the soaking were carried out. The film surface was then observed by SEM. See FIGS. 6A-6C, which show mineral deposition on silk film cast using CRGD-15mer-CDMP1. Deposition began to occur as in round one (FIG. 6B) and continued through round three (FIG. 6C). Control films made from CRGD-15mer and 15mer showed no deposition (data not shown). The SEM images indicate that the functional fusion silk proteins are capable of inducing apatite nucleation and growth, while silk proteins that so not contain mineralizing domains do not promote apatite nucleation.

In addition, scanning electron microscopy (SEM) surface morphologies of recombinant spider silk films after soaking in 1.5×SBF for various periods of time were assessed see FIG. 13. FIG. 13(A1)-FIG. 13(A5), SS15m-CDMP1 soaked in 1.5×SBF for 0, 3, 7, 14 and 21 days: FIG. 13(A1)-FIG. 13(A5) respectively. FIG. 13(B1)-FIG. 13(B5), SS15m soaked in 1.5×SBF for 0,3,7,14 and 21 days: FIG. 13(B1)-FIG. 13(B5) respectively.

The surfaces of the films made of CRGD-15mer-CDMP1 and CRGD-15mer were smooth after casting and there was no big difference between two films (FIG. 13A1 and FIG. 13B1). However, after immersing in 1.5×SBF for 3 days, the surface morphologies of the films were different (FIG. 13A2 and FIG. 13B2).

Transmission Electron Microscopy: The high-magnification image of the crystals grew on CRGD-15mer-CDMP1 (FIG. 14A) showed that the apatite, which appeared flake-like in the SEM, is composed of aggregates of nanocrystals with need shapes 100-200 nm in length. The electron diffraction pattern of the nanocrystals (FIG. 14B) showed diffraction rings. The spacings of the rings agreed with the characteristic x-ray diffraction spacing of hydroxyapatite (JCPDS 09-0432). The rings could be assigned to the (002), (211) planes. In addition, a high-resolution TEM image (FIG. 14C) showed the lattice fringes of apatite nanocrystals. The average distance between fringes is 0.82 nm, which is consistent with the value of teh (100) inerplanar spacing in the apatite structure (0.817 nm).

Example 2 Formation of Silica Silk Fusions Introduction

Silica is widespread in biological systems and serves different functions, among which the most essential ones include support and protection in single-celled organisms, such as diatoms, to higher plants and animals. Despite its widespread occurrence and importance of function, little is known about biosilica and the mechanisms used to produce controlled microscopic and macroscopic silica structures with such nanoscale precision and symmetry. The remarkable control in vivo of the morphology of these beautiful intricate patterns at small length scales are species-specific and has attracted a great deal of interest in recent years as these features exceed the capabilities of present-day synthetic and technological approaches to materials engineering in vitro.

Silaffins^(1; 2; 3; 4; 5) form part of three families of proteins identified to date in the organic matrix of the cell wall of the diatom Cylindrotheca fusiformis. Silaffins are highly post-translationally modified peptides and have received the most attention because of their ability to induce and regulate silica precipitation at ambient temperature and pressure. Silaffins are low-molecular weight polypeptides: natSil-1A (6.5 kDa)⁴, natSil-1B (10 kDa)⁵, and natSil-2 (40 kDa)⁵, isolated by treating the diatom cell wall with ammonium fluoride. A gene sil 1 has been isolated from a C. fusiformis genomic library and it encodes a polypeptide of 265 amino acids. Seven repeated sequences were identified in this sequence and named R1 to R7¹. R1 corresponds to the precursor of Silaffin-1B, while R3 to R7 and R2 correspond to the precursors of the Silaffin-1A1 and Silaffin-1A2, respectively.

Controlled formation of biosilica structures by different proteins and peptides under various physical reaction environments has also been reported^(6; 7; 8; 9). The 19 amino-acid R5 peptide of the Sil1 protein was utilized to obtain silica nanostructures with different morphologies including spheres, arch-shaped morphologies and even elongated fibers⁷. These results suggest that through careful manipulation of the environmental conditions and the application of a linear shear force, distinct morphologies can be attained. These opportunities for control of materials outcomes in biosilica morphology establish important links to device fabrication from these materials with regularity in process and outcomes to achieve technological relevance in the future, such as for silica based micro- and nano-devices.

Applicants have genetically cloned the R5 peptide unit of Sil1 protein to a 15mer spider silk clone of the consensus sequence of the golden orb spider Nephila clavipes, expressed the fusion protein and performed the same silicification reactions on silk films made from the expressed protein. The purpose of fusing this silicification-inducing peptide unit to our genetically engineered silk is to combine the properties of our silk whether in the form of films or other such as spun fibers to the silica precipitating properties of R5 under ambient conditions to produce biomaterials with controlled silica morphologies on the surface. These biomaterials are especially useful in such fields as controlled drug delivery.

Materials and Methods Design and Cloning of Spider Silk Sequences

The repeat unit was selected used in the design of the 15mer and CRGD15mner was based on the consensus sequence of spidroin1 native sequence of Nephila clavipes (accession P19837) (-SGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGT-) (SEQ ID NO: 1). Multimers encoding the repeat were cloned through the transfer of cloned inserts between two shuttle vectors based on pUC 19 and pCR-Script (Novagen, WI), which were ampicillin and chloramphenicol resistant respectively^(10; 11).

The expression vector pET-30a (Novagen, Madison, Wis.) was modified with a linker carrying the SpeI site flanked by sequences encoding the amino acids CRGD (SEQ ID NO: 2) to obtain pET30-link. The two complementary oligonucleotide sequences for the linker were: GGATCCTGTCGCGGTGACACTAGTCGCGGTGACTGTG (SEQ ID NO: 13) and GGATCCACAGTCACCGCGACTAGTGTCACCGCGACAG (SEQ ID NO: 14). The restriction sites of BamHI and SpeI are underlined. The 15mer sequence obtained by multimerization was inserted into pET30-link to generate pET30-CRGD15mer. For the production of a spider silk protein without CRGD (SEQ ID NO: 2), the construct pET30-15mer was obtained by subcloning the NcoI-NotI fragment of pCR-15 into pET-30a vector.

To construct the fusion protein CRGD15mer−R5, the polylinker of pET-21a(+) shuttle vector was redesigned to contain a His-Tag and the CRGD15mer was digested with the restriction enzyme BamHI from pET-30a vector and inserted into the pET-21a(+)-link vector. Two complementary oligonucleotide sequences for the R5 peptide were designed with EcoRI and NotI sites at the 5′ and 3′ ends respectively:

5′ aattcagcagcaaaaaaagcggcagctattcgggcagcaaaggcagcaaacgccgcatcctcgc 3′  (SEQ ID NO: 15)

and

3′ gtcgtcgtttttttcgccgtcgataagcccgtcgtttccgtcgtttgcggcgtaggagcgccgg 5′  (SEQ ID NO: 16)

These synthetic oligonucleotides were annealed and ligated into the pET-21a(+)-link vector right next to the CRGD15mer clone to create the fusion protein with the His-Tag at the C-terminus.

To construct the fusion protein 15mer−R5, the synthetic oligonucleotides were ligated into the NotI and XhoI restriction sites of pET-21a(+) vector right next to the 15mer clone. This fusion protein has a His-Tag at the N-terminus. The amino acid sequence of the R5 peptide of Sil1 protein is: SSKKSGSYSGSKGSKRRIL (SEQ ID NO: 17). Use of other Sil1 protein sequences are also contemplated, for example, the R2 peptide SSKKSGSYSGYSTKKSGSRRIL (SEQ ID NO: 18), the R3 peptide SSKKSGSYSGYSKGSKRRIL (SEQ ID NO: 19), the R4/R6 peptide SSKKSGSYSGYSKGSKRRNL (SEQ ID NO: 20), the R1 peptide SSKKSGSYYSYGTK (SEQ ID NO:21).

Protein Expression and Purification

The constructs pET-21(a)+-15mer−R5 and pET-30a-CRGD15mer−R5 were used to transform the E. coli host strains RY-3041, a mutant strain defective in the expression of SlyD protein, for expression^(12; 13). Cells were cultivated in LB broth at 37° C. Protein expression was induced by the addition of 1 mM IPTG (Fisher Scientific, Hampton, N.H.) when the OD₆₀₀ was between 0.6 and 0.8. After approximately 6 hours of protein expression, the cells were harvested by centrifugation at 9500 rpm. For large scale expression, E. coli was grown in a fermentor (Bioflo 3000, New Brunswick Scientific Co., Edison, N.J.) in minimal medium supplemented with 1% yeast extract. Ammonia was used as the base to maintain the pH at 6.8. When the pH exceeded 6.88, as a result of glucose exhaustion in the culture, a feed solution (50% glucose, 10% Yeast Extract, 2% MgSO₄.7H₂O) was added. Pure O₂ was also provided to the culture to sustain the level of dissolved oxygen above 40%. All culture media contained kanamycin (50 μg/ml) or ampicillin (100 μg/ml) for selectivity. For the fermentor grown cells, expression was induced when the absorbance was between 25 and 30 at OD₆₀₀.

The cell pellets were resuspended by adding denaturing buffer (100 mM NaH₂PO₄, 10 mM Tris HCl, 8 M urea, pH 8.0) containing 10 mM imidazole. The cells were lysed by stirring for 30 min and were then centrifuged at 9500 rpm at 4° C. for 30 min. His-tag purification of the proteins was performed by addition of Ni-NTA agarose resin (Qiagen, Valencia, Calif.) to the supernatant (batch purification) under denaturing conditions. After washing the column with denaturing buffer at pH 6.3, the proteins were eluted with denaturing buffer at pH 4.5 (without imidazole).

SDS-polyacrylamide gel electrophoresis (PAGE) was performed using 4-12% precast NuPage Bis-Tris gels (Invitrogen, Carlsbad, Calif.). Electrophoresis was performed in MOPS buffer for 50 min at 200V. Purified samples were extensively dialyzed against several changes of H₂O. For dialysis, Snake Skin membranes (Pierce, Rockford, Ill.) with MWCO of 7000 or lower were used. The dialyzed samples were lyophilized using a LabConco lyophilizer. For determination of the amino acid composition, the samples were submitted to the W. M. Keck Foundation Bioteclnology Resource Laboratory (Yale University, New Haven, Conn.). The samples were analyzed from bands of interest cut out from the gel or lyophilized powder after purification and dialysis. Determination of protein concentration was performed by BCA assay (Pierce, Rockford, Ill.) or the molar absorptivity at 280 nm. FIGS. 7A and 7B show the sequence of CRGD15mer−R5 (FIG. 7A) and 15mer−R5 (FIG. 7B)

Silicification Reactions

The lyophilized fusion proteins were then dissolved in HFIP solvent at a concentration of 2.5% w/v overnight at 4° C. The silk-HFIP solutions were then pipetted into 24-well culture plates and left in the hood for about 3 hours for the silk films to form and dry out. The silk films were then either left as is or treated with 90% v/v methanol to induce β-sheet formation and prevent resolubilization of the films in aqueous solutions. 100 mM phosphate buffer at pH 5.5 was added to the silk films and leave to sit for about 30 minutes before 1M tetramethoxysilane (TMOS) dissolved in 1 mM hydrochloric acid was added to the mixture for about 10 minutes. The films were then washed with MilliQ water three times and left to dry overnight in the fume hood. These films were then analyzed using a LEO 982 Scanning Electron Microscope at the CIMS facility at Harvard University.

SEM Results

In order to exploit the self-assembling properties of silk in developing silk-silica nanocomposites, experiments were performed using TMOS alone as the precursor. Four genetically engineered variants of the spider silk protein (two controls, one with and one without RGD but both without R5, two chimeric versions of the controls but with R5) were cast into films that were either left untreated or were treated with methanol to induce a structural transition to β-sheet and thus decrease film solubility in aqueous buffer. Silicification reactions were performed on the films yielding spherical silica structures with diameters ranging from ˜0.5 to 2.0 μm only when the silica precipitating domain, R5 peptide, was fused to the C-terminus of the silk proteins (FIG. 8). The silk proteins that did not contain R5, CRGD15mer and 15mer, did not yield significant changes in surface morphology of the films upon exposure to the silicification reactions.

Fusion proteins were assembled into fibers by electrospinning. SEM images of the electropun fibers formed from the chimeric proteins (FIGS. 9A-9D), and the morphological characteristics observed when the fibers were treated with methanol, were similar to those we observed previously for electrospun silk fibroin with polyethylene oxide¹⁵. Upon silicification on electrospun mats formed from the chimeric protein CRGD15mer+R5 without methanol treatment, similar spherical silica structures were observed as in the reactions on the cast films. However, the dimensions of the silica spheres were slightly smaller, ranging from 200 to 400 nm (FIG. 9). When the electrospun fibers consisting of the chimera CRGD15mer+R5 were not treated with methanol, the fibers fused together on the surface. Without the β-sheet inducing methanol treatment, the fibers are prone to partially solubilize on the surface yielding a thin film upon which the mineralization reaction takes place. However, upon treatment of the chimera CRGD15mer+R5 electrospun mats with methanol before silicification, a thin film formed from the solubilized and then fused fibers at the surface and silica nanospheres did not form as in the sample above. Instead, the fibers fused with each other as expected¹⁵ and although mineralization occurred as confirmed by XPS, silica deposited around the fibers providing a non-uniform coating instead of the spheres (FIG. 9). When the chimera CRGD15mer+R5 was electrospun during the silica polymerization process (concurrent processing), silica deposition was induced in and on the fibers and elliptically shaped silica particles fused to the fibers were observed (FIG. 9). XPS analysis of the resulting fibers confirmed the presence of elemental silicon (FIG. 10). Thus, the concurrent processing approach, fiber spinning and silicification reactions, resulted in a different morphology of the silica in terms of location within the fibers and shape, compared to the silicification reactions conducted post electrospinning.

Sequences

Fibrous protein domain sequence derived from Spidroin1 (the native sequence of the goldon orb spider Nephilia clavipes) SGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGT (SEQ ID NO: 1)

A linker sequence CRGD (SEQ ID NO: 2)

The consensus fibrous protein domain sequence derived from Spidroin1 (the native sequence of the goldon orb spider Nephilia clavipes) with CRGD linker CRGDTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGT (SEQ ID NO: 3)

15 mer: An amino acid sequence that contains a repeat of a fibrous protein domain sequence derived from Spidroin1 (the native sequence of the goldon orb spider Nephilia clavipes) The fibrous protein domain sequence is repeated 15 times. MASMTGGQQMGRGSAMASGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTS GRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGG AGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRG GLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQ GGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGG QGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYG GLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAG AAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGS QGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAA AAGGAGQGGYGGLG SQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGT SHHHHH (SEQ ID NO: 4)

CRGD-15mer: an amino acid sequence that contains a repeat of a fibrous protein domain sequence derived from Spidroin1 (the native sequence of the goldon orb spider Nephilia clavipes) The fibrous protein domain sequence is repeated 15 times with a CRGD linking sequence MASMTGGQQMGRGSCRGDTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQG TSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAA GGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSG RGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGA GQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGL GGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGG YGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQG AGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGL GSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGA AAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQ GTSRGDCGSHHHHHH (SEQ ID NO: 5)

CDMP1, the C-Terminal Sequence of DMP1, a Mineralizing domain RGDNPDNTSQTGDQRDSESSEEDRLNTFSSSESQSTEEQGDSESNESLSLSEESQES AQDEDSSSQEGLQSQSASRESRSQESQSEEDSRSEENRDSDSQDSSRSKEESNSTG STSSSEEDNHPKNIEADNRKLIVDAYHNKPIGDQDDNDCQDGY (SEQ ID NO: 6)

DMP1, the full length sequence of DMP1, a Mineralizing domain LPVARYQNTESESSEERTGNLAQSPPPPMANSDHTDSSESGEELGSDRSQYRPAG GLSKSAGMDADKEEDEDDSGDDTFGDEDNGPGPEERQWGGPSRLDSDEDSADT TQSSEDSTSQENSAQDTPSDSKDHHSDEADSRPEAGDSTQDSESEEYRVGGGSEG ESSHGDGSEFDDEGMQSDDPGSTRSDRGHTRMSSADISSEESKGDHEPTSTQDSD DSQDVEFSSRKSFRRSRVSEEDDRGELADSNSRETQSVSTEDFRSKEESRSETQED TAETQSQEDSPEGQDPSSESSEEAGEPSQESSSESQEGVASESRGDNPDNTSQTGD QRDSESSEEDRLNTFSSSESQSTEEQGDSESNESLSLSEESQESAQDEDSSSQEGLQ SQSASRESRSQESQSEEDSRSEENRDSDSQDSSRSKEESNSTGSTSSSEEDNHPKNI EADNRKLIVDAYHNKPIGDQDDNDCQDGY (SEQ ID NO: 7)

BSP: Bone sialoprotein, a Mineralizing domain SEFPVQSSSDSSEENGNGDSSEEEEEEEENSNEEENNEENEDSDGNED (SEQ ID NO: 8)

CRGD-15mer-CDMP1: (SEQ ID NO: 9): a fusion protein of (SEQ ID NO: 5) and (SEQ ID NO: 6) MASMTGGQQMGRCRGDTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTS GRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGG AGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRG GLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQ GGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGG QGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYG GLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAG AAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGS QGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAA AAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGT SRGDCGSRGDNPDNTSQTGDQRDSESSEEDRLNTFSSSESQSTEEQGDSESNESLS LSEESQESAQDEDSSSQEGLQSQSASRESRSQESQSEEDSRSEENRDSDSQDSSRSK EESNSTGSTSSSEEDNHPKNIEADNRKLIVDAYHNKPIGDQDDNDCQDGY (SEQ ID NO: 9)

CRGD-15mer-DMP1 (SEQ ID NO: 10): a fusion protein of(SEQ ID NO: 5) and SEQ ID NO: 7) MASMTGGQQMGRCRGDTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTS GRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGG AGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRG GLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQ GGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGG QGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYG GLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAG AAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGS QGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAA AAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGT SRGDCGSLPVARYQNTESESSEERTGNLAQSPPPPMANSDHTDSSESGEELGSDRS QYRPAGGLSKSAGMDADKEEDEDDSGDDTFGDEDNGPGPEERQWGGPSRLDSD EDSADTTQSSEDSTSQENSAQDTPSDSKDHHSDEADSRPEAGDSTQDSESEEYRV GGGSEGESSHGDGSEFDDEGMQSDDPGSTRSDRGHTRMSSADISSEESKGDHEPT STQDSDDSQDVEFSSRKSFRRSRVSEEDDRGELADSNSRETQSVSTEDFRSKEESR SETQEDTAETQSQEDSPEGQDPSSESSEEAGEPSQESSSESQEGVASESRGDNPDN TSQTGDQRDSESSEEDRLNTFSSSESQSTEEQGDSESNESLSLSEESQESAQDEDSS SQEGLQSQSASRESRSQESQSEEDSRSEENRDSDSQDSSRSKEESNSTGSTSSSEED NHPKNIEADNRKLIVDAYHNKPIGDQDDNDCQDGY (SEQIDNO:10)

15mer-BSP (SEQ ID NO: 11): a fusion protein of (SEQ ID NO: 4) and (SEQ ID NO: 8) MASMTGGQQMGRGSAMASGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTS GRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGG AGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRG GLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQ GGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGG QGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYG GLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAG AAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGS QGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAA AAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGT SEFPVQSSSDSSEENGNGDSSEEEEEEEENSNEEENNEENEDSDGNEDKLHHHHH H (SEQ ID NO: 11)

CRGD-15mer-BSP (SEQ ID NO: 12): a fusion protein of (SEQ ID NO: 5) and (SEQ ID NO: 8) with linker MASMTGGQQMGRGSCRGDTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQG TSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAA GGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSG RGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGA GQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGL GGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGG YGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQG AGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGL GSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGA AAAAGGAGQGGYGGLGSQGTSGRGGLGGQGAGAAAAAGGAGQGGYGGLGSQ GTSRGDCGSESEFPVQSSSDSSEENGNGDSSEEEEEEEENSNEEENNEENEDSDGN EDKLHHHHHH (SEQ ID NO: 12)

One of two complementary oligonucleotide sequences for the linker carrying the SpeI site flanked by sequences encoding the amino acids CRGD to obtain pET30-link GGATCCTGTCGCGGTGACACTAGTCGCGGTGACTGTG (SEQ ID NO: 13)

One of two complementary oligonucleotide sequences for the linker carrying the SpeI site flanked by sequences encoding the amino acids CRGD to obtain pET30-link GGATCCACAGTCACCGCGACTAGTGTCACCGCGACAG (SEQ ID NO: 14)

One of two complementary oligonucleotide sequences for the R5 peptide (contains the 19 amino-acid R5 peptide of the Sil1 protein) which were designed with EcoRI and NotI sites at the 5′ and 3′ ends respectively

5′ aattcagcagcaaaaaaagcggcagctattcgggcagcaaaggcagcaaacgccgcatcctcgc 3′ (SEQ ID NO: 15)

One of two complementary oligonucleotide sequences for the R5 peptide (contains the 19 amino-acid R5 peptide of the Sil1 protein) which were designed with EcoRI and NotI sites at the 5′ and 3′ ends respectively.

3′ gtcgtcgtttttttcgccgtcgataagcccgtcgtttccgtcgtttgcggcgtaggagcgccgg 5′ (SEQ ID NO: 16)

The amino acid sequence of the R5 peptide of Sil1 protein from C. fusiformis is: SSKKSGSYSGSKGSKRRIL (SEQ ID NO: 17)

The amino acid sequence of the R2 peptide of Sil1 protein from C. fusiformis is: SSKKSGSYSGYSTKKSGSRRIL (SEQ ID NO: 18),

The amino acid sequence of the R3 peptide of Sil1 protein from C. fusiformis is: SSKKSGSYSGYSKGSKRRIL (SEQ ID NO: 19),

The amino acid sequence of the R4/R6 peptide of Sil1 protein from C. fusiformis is: SSKKSGSYSGYSKGSKRRNL (SEQ ID NO: 20).

The amino acid sequence of the R1 peptide of Sil1 protein from C. fusiformis is: SSKKSGSYYSYGTKK (SEQ ID NO:21).

Bone Sialoprotein Precursor [Homo sapiens] Accession AAC95490 is: MKTALILLSI LGMACAFSMK NLHRRVKIED SEENGVFKYR PRYYLYKHAY FYPHLKRFPV QGSSDSSEEN GDDSSEEEEE EEETSNEGEN NEESNEDEDS EAENTTLSAT TLGYGEDATP GTGYTGLAAI QLPKKAGDIT NKATKEKESD EEEEEEEEGN ENEESEAEVD ENEQGINGTS TNSTEAENGN GSSGGDNGEE GEEESVTGAN AEGTTETGGQ GKGTSKTTTS PNGGFEPTTP PQVYRTTSPP FGKTTTVEYE GEYEYTGVNE YDNGYEIYES ENGEPRGDNY RAYEDEYSYF KGQGYDGYDG QNYYHHQ (SEQ ID NO: 22)

Silaffin 1 precursor (natSil-1) [Contains: Silaffin-1B; Silaffin-1A2; Silaffin-1A1] Accession Q9SE35: MKLTAIFPLL FTAVGYCAAQ SIADLAAANL STEDSKSAQL ISADSSDDAS DSSVESVDAA SSDVSGSSVE SVDVSGSSLE SVDVSGSSLE SVDDSSEDSE EEELRILSSK KSGSYYSYGT KKSGSYSGYS TKKSASRRIL SSKKSGSYSG YSTKKSGSRR ILSSKKSGSY SGSKGSKRRI LSSKKSGSYS GSKGSKRRNL SSKKSGSYSG SKGSKRRILS SKKSGSYSGS KGSKRRNLSS KKSGSYSGSK GSKRRILSGG LRGSM (SEQ ID NO: 23)

CRGD15mer−R5: Sequence of fusion protein; fusion of CRGD15mer to the amino acid sequence of the R5 peptide of Sil1 protein from C. fusiformis. MASMTGGQQM GRGSCRGDTS GRGGLGGQGA GAAAAAGGAG QGGYGGLGSQ GTSGRGGLGG QGAGAAAAAG GAGQGGYGGL GSQGTSGRGG LGGQGAGAAA AAGGAGQGGY GGLGSQGTSG RGGLGGQGAG AAAAAGGAGQ GGYGGLGSQG TSGRGGLGGQ GAGAAAAAGG AGQGGYGGLG SQGTSGRGGL GGQGAGAAAA AGGAGQGGYG GLGSQGTSGR GGLGGQGAGA AAAAGGAGQG GYGGLGSQGT SGRGGLGGQG AGAAAAAGGA GQGGYGGLGS QGTSGRGGLG GQGAGAAAAA GGAGQGGYGG LGSQGTSGRG GLGGQGAGAA AAAGGAGQGG YGGLGSQGTS GRGGLGGQGA GAAAAAGGAG QGGYGGLGSQ GTSGRGGLGG QGAGAAAAAG GAGQGGYGGL GSQGTSGRGG LGGQGAGAAA AAGGAGQGGY GGLGSQGTSG RGGLGGQGAG AAAAAGGAGQ GGYGGLGSQG TSGRGGLGGQ GAGAAAAAGG AGQGGYGGLG SQGTSRGDCG SEFSSKKSGS YSGSKGSKRR ILCGRHHHHH H (SEQ ID NO: 24)

15mer−R5: Sequence of fusion protein; fusion of 15mer to the amino acid sequence of the R5 peptide of Sil1 protein from C. fusiformis. MHHHHHHSSG LVPRGSGMKE TAAAKFERQH MDSPDLGTDD DDKAMASGRG GLGGQGAGAA AAAGGAGQGG YGGLGSQGTS GRGGLGGQGA GAAAAAGGAG QGGYGGLGSQ GTSGRGGLGG QGAGAAAAAG GAGQGGYGGL GSQGTSGRGG LGGQGAGAAA AAGGAGQGGY GGLGSQGTSG RGGLGGQGAG AAAAAGGAGQ GGYGGLGSQG TSGRGGLGGQ GAGAAAAAGG AGQGGYGGLG SQGTSGRGGL GGQGAGAAAA AGGAGQGGYG GLGSQGTSGR GGLGGQGAGA AAAAGGAGQG GYGGLGSQGT SGRGGLGGQG AGAAAAAGGA GQGGYGGLGS QGTSGRGGLG GQGAGAAAAA GGAGQGGYGG LGSQGTSGRG GLGGQGAGAA AAAGGAGQGG YGGLGSQGTS GRGGLGGQGA GAAAAAGGAG QGGYGGLGSQ GTSGRGGLGG QGAGAAAAAG GAGQGGYGGL GSQGTSGRGG LGGQGAGAAA AAGGAGQGGY GGLGSQGTSG RGGLGGQGAG AAAAAGGAGQ GGYGGLGSQG TSSSKKSGSY SGSKGSKRRI L (SEQ ID NO: 25)

PCR PRIMER BamH I site, C-DMP1f: 5′-CAGGATCCAGGGGTGACAACCCAGAT -3′ (SEQ ID NO: 26)

PCR PRIMER Xho I site, C-DMP1r: 5′ GCCTCGAGGTAGCCATCTTGGCAATC-3′ (SEQ ID NO: 27).

The references cited herein and throughout the application are incorporated by reference.

REFERENCES

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1. A fusion polypeptide comprising a fibrous protein domain and a mineralizing domain.
 2. The fusion polypeptide of claim 1, wherein the fibrous protein domain is obtained from silk, collagens, coiled-coiled leucine zipper proteins, elastins, keratins, actins, or tubulins.
 3. The fusion polypeptide of claim 2, wherein the fibrous protein domain comprises an amino acid sequence from the silk protein Spidroin
 1. 4. The fusion polypeptide of claim 3, wherein the an amino acid sequence from the silk protein Spidroin 1 is (SEQ ID NO: 1).
 5. The fusion polypeptide of claim 3, wherein the an amino acid sequence from the silk protein Spidroin 1 is (SEQ ID NO: 3).
 6. The fusion polypeptide of claim 1, wherein the fibrous protein domain is (SEQ ID NO: 4).
 7. The fusion polypeptide of claim 1, wherein the fibrous protein domain is (SEQ ID NO: 5).
 8. The fusion polypeptide of claim 1, wherein the mineralizing domain induces the formation of hydroxyapatite, silica, cadmium sulfide or magnetite.
 9. The fusion polypeptide of claim 1, wherein the mineralization domain is obtained from dentin matrix protein 1 (DMP1), bone sialoprotein (BSP), or silaffin-1 (Sil1) protein.
 10. The fusion polypeptide of claim 10, wherein the mineralization domain is derived from dentin matrix protein 1 (DMP1) and is (SEQ ID NO: 6) or (SEQ ID NO: 7).
 11. The fusion polypeptide of claim 10, wherein the mineralization domain is derived from bone sialoprotein (BSP) and is (SEQ ID NO: 8).
 12. The fusion protein of claim 1, where the mineralizing domain is selected from the group consisting of the 19 amino-acid R5 peptide of the Sil1 protein (SEQ ID NO: 17), the R2 peptide of the Sil1 protein (SEQ ID NO: 18), the 19 amino-acid R3 peptide of the Sil1 protein (SEQ ID NO: 19), the 19 amino-acid R6 peptide of the Sil1 protein (SEQ ID NO: 20) and the 15 amnino-acid R1 peptide of the Sil1 protein (SEQ ID NO: 21).
 13. A fusion polypeptide comprising (SEQ ID NO: 9).
 14. A fusion polypeptide comprising (SEQ ID NO: 10).
 15. A fusion polypeptide comprising (SEQ ID NO: 11).
 16. A fusion polypeptide comprising (SEQ ID NO: 12).
 17. A fusion polypeptide comprising a fusion of the 15 mer silk fibrous protein domain (SEQ ID NO: 4) and the R5 peptide of the Sil1 protein (SEQ ID NO: 17).
 18. A fusion polypeptide comprising a fusion of the CRGD-15 mer silk fibrous protein domain (SEQ ID NO: 5) and the R5 peptide of the Sil1 protein (SEQ ID NO: 17).
 19. An isolated nucleic acid encoding the fusion protein of any of claims 1-18.
 20. A method for forming a fibrous protein inorganic-composite comprising: (a) contacting a fusion protein of any of claims 1-18 with an inorganic material capable of mineralizing for a sufficient period of time to allow mineralization of the inorganic material.
 21. The method of claim 20 wherein the inorganic material forms hydroxyapatite or silica.
 22. The method of claim 20, wherein the fusion protein comprises a fiber, film, or sponge.
 23. The method of claim 21, wherein the fiber, film, or sponge further comprises an agent.
 24. The method of claim 22, wherein the agent is selected from the group consisting of a protein, peptide, nucleic acid, PNA, aptamer, antibody and a small molecule.
 25. A product produced by the method of claim
 20. 